Distortion

The Zeiss Distagon 25mm f/2.8 produces a moderate degree (~1.7%) of barrel distortion. This is about average for a prime lens in this class.

The chart above has a real-world size of about 120x80cm.

Vignetting

The vignetting characteristic seems to be a real weakness of Zeiss Z-series lenses when used on full format DSLRs. The Distagon shows a fairly extreme deterioration of ~1.9EV at f/2.8 - this will be visible in most scenes. The problem is still very pronounced at f/4 (@ ~1EV) but it's not overly significant anymore from f/5.6 onwards.

We're performing our vignetting analysis based on
(uncorrected) JPEGs straight from the camera. The JPG engine of the Nikon D3x features a rather flat
gradation curve, thus has a moderate contrast characteristic, resulting in comparatively low vignetting figures - the
corresponding Canon figures are roughly 40% higher due to the more
aggressive default contrast setting.

MTF (resolution)

The Zeiss lens produced disappointing resolution figures in our MTF tests. The center performance is great straight from f/2.8 till about f/8. However, the borders/corners are rather soft at large apertures. They improve to good level at f/5.6 but the generally very good performance peak is not reached prior of f/8-f/11. This is very "late" for a prime lens and the figures aren't all that impressive.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Lateral Chromatic aberrations (color shadows at harsh contrast transitions) are low with below 1px on the average at the image borders. This is usually not field relevant anymore.